Which major event occurred during the Precambrian time? Landmasses were all below water.

Cyanobacteria filled the water and air with oxygen.

Trilobites became extinct.

People lived on Earth.

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Dr. redbull conducts a study to determine whether her new energy drink children perform better on spelling tests. she randomly assigns children to one of two groups: group 1 receives her energy drink and group 2 receives water. she then gives both groups a spelling test and compares the results. which of the following is true about this study? a. group 1 is the dependent variable and group 2 is the independent variable. b. this is a correlational study. c. group 2 is the experimental group. d. the independent variable is the type of drink the children receive.

The economy of the united states is best described as